2017ApJ...846...20L

We identify sources with extremely hard X-ray spectra (i.e., with photon indices of Γ0.6) in the 13 deg² NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at 10 keV. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5-24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; N_H 1.5×10²⁴ cm^–2) AGNs at low redshift (z 0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy ( 10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray-mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10-40 keV luminosities of the extreme sources cover a broad range, from ~5×10⁴² to 10⁴⁵ erg s^–1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z 0.07), where we measure a high CT fraction of f_CT^obs=30_–12⁺¹⁶ % . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of "normal" AGNs.